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Category Archives: Geology

From Burbank Firefighters Local 778, a group of whom were trapped in the Deer Canyon area until the landslide subsided:

Two people were in the car and survived. They made a beeline out of there after an evacuation order said the basin above them might be overtopped. It was, and they hydroplaned with the debris flow down the hill, then regained control and went up another road to escape the flow.

The debris basin that was inundated, Upper Sunset, is at the upper right. The car came down Country Club Drive, which emanates from the Sunset Debris Basin access road.

The debris basin after the landslide is below. The wall appears to have been breached but was not. There’s ongoing construction to raise the rim five feet to increase the capacity of the basin by 8,000 cubic yards – see the scaffolding – and that middle portion is not yet started.

The prevailing term in government warnings and the news reports out of Santa Barbara County is mudslide, not landslide. Yes, it is primarily mud by volume, but mudslide seems far too mild a term to me when I look at these pictures from today in the northern part of Montecito, just south of the Santa Ynez range that’s northeast of Santa Barbara.

This sort of confluence of events occurs just once or twice a decade at this scale, with multiple fatality landslides occurring every second or third decade. Wouldn’t some people – specifically those who haven’t seen this before – think of mudslide warnings, “Oh, some mud? Whatever”? I doubt anyone would think that if the more apt “landslide” was used instead. A reaction of “Let’s get the hell out of here” would be a little more likely, I think – and a lot more sensible.

You may have heard that, after the California wildfires, there’s a scramble to empty “debris basins” there because so many hillsides now lack any vegetation to halt or even slow landslides that will occur on burn-scarred hills after heavy rainfall. The debris basins are man-made bowls of varying sizes, whose construction first began in 1915, meant to catch landslide debris but allow water through. There are more than 120 of them near Los Angeles, where they’re essential because the tectonically active San Gabriel Mountains are disintegrating at one of the fastest rates on the planet. The basins are regularly emptied by crews, but because of the volume of material, it can be difficult to keep up.

The regional forecast for the next four days is not good: 1-2 inches of rain at the coast and up to 5 inches on west-facing slopes. As little as a quarter-inch of rain in an hour is capable of triggering a landslide in burned areas.

John McPhee wrote about the basins and the underlying geology of the region in The New Yorker in 1988. The second half of his essay is behind their paywall, but the fascinating first half is freely readable here:

Both parts are included in his book, The Control of Nature. When he wrote that essay thirty years ago, the basins had already collected – and been emptied of – over twenty million tons of landslide material. In the eyebrow-lifting words of McPhee: “Some of it is Chevrolet size. Boulders bigger than cars ride long distances in debris flows.”

The lecture notes about McPhee’s essay on this page summarize well the never-ending chaparral overgrowth/wildfire/rain/landslide cycle in Southern California.

“The low stuff, at the buckwheat level, is often called soft chaparral. Up in the tough chamise, closer to the lofty timber, is high chaparral, which is also called hard chaparral. High or low—hard, soft, or mixed—all chaparral has in common an always developing, relentlessly intensifying, vital necessity to burst into flame. In a sense, chaparral consumes fire no less than fire consumes chaparral.”

As a side note, there’s no need to find blame in campfires of the homeless, because Southern California wildfires are inevitable. They would, with 100% certainty, occur even if the region was completely uninhabited. It’s not a matter of if – it’s a matter of when.

The strangest aspect of the basins is where much of their debris is transported once removed: back up into the mountains. McPhee called this bizarre flood control district job security an “elegant absurdity”.

Traveling from the west in the area of the record-breaking Thomas Fire to the east, here are the Santa Barbara County debris basins:

Then Ventura County’s – a little rough looking because I cobbled this together from four zone maps of differing scales:

And finally, Los Angeles County’s debris basins, where you can easily see that the landslide problem is most acute:

This map of likely landslide paths after the Station Fire in 2009 is an example of just how acute. This area is near the centre of the map above and not far north-northeast of Burbank, Glendale, and Pasadena:

Now is a good time to watch Professor Iain Stewart’s excellent four-part BBC documentary “Journeys into the Ring of Fire” again. Of particular interest this week is part 2, the only one that seems to be on YouTube. The others can be found at MVGroup – you can get there from the page linked above.

While most of the series focuses on the geology, volcanology, and plate tectonics of the Pacific Rim, this California episode includes a discussion of the extreme and inevitable fire risks – some avoidable but not avoided – inherent in parts of Southern California. That segment starts at 42:15, but I think you won’t be disappointed if you watch the whole episode.